A neutron star is the core left behind after a massive star explodes. Its matter is so compressed that a teaspoonful would weigh billions of tons. But the exact size and mass of such objects remain debated. A team of astronomers studied the system EXO 0748-676, where a neutron star pulls matter from a normal companion star. It occasionally flares up and then quiets down. During quiescent periods, its surface cools, and X-ray emission can reveal key parameters.

The researchers gathered 20 observations from 2008 to 2025—14 from Chandra and 6 from XMM-Newton. They ran the data through a hydrogen atmosphere model and applied a Markov Chain Monte Carlo method. They found the star's mass to be 1.77 solar masses and its radius 12.62 km. That's like squeezing one and a half Earths into a ball the size of a small city. These numbers favor a fairly stiff equation of state — meaning the matter inside doesn't compress easily.

They also compared two quiescent periods — before and after the 2024–2025 outburst. After the new flare, the star's crust began cooling again, and they could track the process. This is a rare chance to see a neutron star recover from violent accretion.